Fiber-reinforced carbon and graphite articles and method for the production thereof

ABSTRACT

The present invention relates to the fabrication of thick, three-dimensional structures comprising discontinuous thermoset pitch fiber, and to composites comprising such structures embedded in a matrix material such as a thermoset resin, ceramic, metal or carbon. Carbon-carbon fiber composite articles produced from porous carbon preforms obtained by carbonizing such structures followed by infiltration with pyrolytic carbon using CVD processes exhibit surprisingly high through-thickness thermal conductivity and isotropic properties.

BACKGROUND OF THE INVENTION

This invention relates to carbon fiber-reinforced composites, and moreparticularly to composites composing carbon fiber reinforcement embeddedin a carbon, thermoset resin, metal or ceramic matrix. The compositesare formed by infiltrating a porous preform comprising carbonized pitchfiber with the matrix component or a precursor thereof, whereby thematrix component is deposited throughout the structure. Carbon-carbonfiber composites exhibiting unusually high thermal conductivity may beformed by depositing carbon within the interstices of the porous preformby in filtration of the preform using known carbon vapor depositiontechniques, or by impregnation of the preform with pitch or acabonizable resin which then is cured and carbonized. Further heattreatment of the carbon-carbon fiber composites may be used tographitize the carbon components. As used herein, the term "carbon" isintended to include both ungraphitized and graphitized carbon. Thus,carbon fiber preforms and reinforcement may comprise graphitized,partially graphitized or ungraphitized carbon reinforcing fibers or amixture thereof, and carbon-carbon fiber composites comprising suchreinforcement embedded in a matrix of graphitized, partially graphitizedor ungraphitized carbon.

The present invention is particularly concerned with carbon-carbon fibercomposites intended for use in applications where severe shear stresseswill be encountered, for example, by being subjected to circumferentialstress. A prime example of such use is a friction disc employed in adisc brake. Such discs are essentially annular in shape, having at leastone surface of each disc being provided with a friction-bearing surface.Braking is accomplished through contact between the friction-bearingsurfaces of the discs, thereby converting the mechanical energy of therotating portion of the brake to heat. In addition to withstanding theshearing stresses, the discs thus also are required to act as heatsinks, dissipating high heat loads. Because of its strength, density,heat capacity, thermal conductivity, coefficient of friction andstability to its sublimation temperature (about 3600° C.), carbon hasbeen particularly attractive for use in constructing such disc brakes,particularly where weight is a major consideration such as in aircraft.

In the prior art, composites have generally been fabricated by orientingor directionally aligning the carbon fiber component, which generallyhas been thought necessary in order to take advantage of fiber strengthand enhance mechanical properties of the composite. Fabricating thecomposite with the desired fiber orientation is more readilyaccomplished by use of continuous carbon fiber, and such fiber has beenpreferred over discontinuous fiber for these applications. The primaryforms of continuous fiber employed in composite fabrication includewoven textile fabric or unidirectional tapes for use in lay-upstructures, and continuous fiber yarn or tow, which are used forfilament winding and in braided structures. For example, in acommonly-used process for producing carbon composite brake components,annuli are cut from sheets of PAN-based graphite cloth or unidirectionaltape, coated with a suitable binder, stacked and then heated suitably tocarbonize the binder. However, variations in binder thickness lead touneven expansion and contraction during the curing and the resultingcomposite has set up within it internal stresses which may causecracking and stress failure in use. Alternative processes designed toovercome such problems are also widely used. For example, a layeredstack formed from dry fabric annuli may be infiltrated withvapor-deposited carbon to bind the carbon fibers into a rigid structuresuitable for impregnation with a carbonizable binder.

Prior art structures formed from stacked fabric or the like necessarilyhave the reinforcing fiber distributed within and aligned along each ofthe planes formed by the fabric layer. The interlayer spaces, lackingfiber reinforcement, generally exhibit lower strength than the fabriclayers. Some form of reinforcement is thus needed to improve interlayerstrength and thereby avoid or reduce failure through delamination.

Needlepunching is widely used in the textile arts to strengthen stackedfabric structures and improve structural integrity. Generally described,needlepunching operations are carded out by forcing barbed needlesnormally through the stack layers in the thickness direction. A portionof the fiber within the fabric layers is gathered by the barbs andrepositioned in the thickness direction, reinforcing the individualfabric layers as well as the stack. The fiber making up the layers iscontinuous, hence the needlepunching operation necessarily breaksindividual filaments when re-orienting them. To avoid or at leastminimize such breakage, improved processes wherein staple fiber isincluded within the structure, either as part of the fabric layer or asalternating layers of staple fiber sheet, have been used to supplystaple fiber to the needles for re-orienting in the needlepunchingoperation. Needlepunching operations have been employed in the art withcarbon fiber sheet and tape to provide preform structures having goodintegrity for use in the production of carbon-carbon fiber reinforcedcomposites.

As noted, uniformity in the carbon-carbon fiber composite structure hasbeen considered important to the integrity and strength of the product,and the art has continually attempted to develop improved methods forproviding uniformity in the preform component. Uniform needlepunching,both in terms of evenly-spaced needles and controlled depth of theneedlepunching, has been thought to be important to the uniformity ofthe product. One widely accepted approach to achieving a high degree ofcontrol in constructing preforms from layered fabric or tape has been toneedle each of the layers to the layer below as it is added. Prior artmethods, such as those disclosed in U.S. Pat. No. 4,621,662 and in U.S.Pat. No. 4,955,123, take great care to emphasize the importance of usingjust such needlepunching procedures, even to the extent of applying theneedlepunching to the fabric at the point of contact with the underlyinglayer as the fabric is wound on a mandrel. More recently, in U.S. Pat.No. 5,217,770, there is disclosed a process of forming a braided,continuous tube from continuous fiber tow or yam which then is flattenedinto a tape and layered to form an annular structure, each layer beingneedle punched as it is added.

The needlepunching process has also been applied to layering fabricsheet formed of carbon fiber and coated with carbonizable binders, whichmay include conductive particulate or fibrous filler. Needlepunching thestack is said to aid penetration of the liquid binder into theinterstices of the fabric layers. Distributing the binder and the carbonfiber in the thickness direction by needlepunching provides, after acarbonizing step, reinforcement of the carbon matrix which may improveresistance to delamination.

The needled preform structures are used as substrates for depositingcarbon matrix material, thereby providing reinforcement for the matrixcarbon in the carbon-carbon fiber reinforced composite. Known vapordeposition techniques may be used to infiltrate and deposit pyrolyticcarbon on the fibrous carbon skeleton. Chemical vapor deposition ofcarbon and impregnation with carbonizable binders have also been used incombination. A substrate formed of layers of fibrous carbon fabric orsimilar material thus may first be infiltrated with vapor-depositedcarbon to bond the fibrous materials, then impregnated with carbonizablefiller material, cured and carbonized to provide the dense,fiber-reinforced carbon article. These and other processes are wellknown and widely disclosed in the art.

As noted, the high degree of fiber alignment within the structure ofthese prior art composites is intended to take advantage of the strengthand dimensional stability of the carbon fiber. However, compositeshaving the entire fiber content aligned in a single direction wouldnecessarily be highly anisotropic in character, exhibiting a high degreeof strength and dimensional stability in the fiber direction whilesuffering greatly reduced strength properties and poor dimensionalstability in the transverse direction. To ensure that the strength ofthe composite, as well as its heat transfer characteristics and otherimportant mechanical properties, will be reasonably uniform and tominimize unidirectional shrinkage which may cause warping anddistortion, the fiber direction will be varied throughout the structure,imparting some isotropic character to the composite. When using fabricor the like, the fabricator has had to resort to varying fiberorientation between successive layers of the structure, for example,using radial orientation in one layer, chordal in the next, and so on,thereby providing a composite having characteristics termedquasi-isotropic. As described above, three-dimensional weaving,needlepunching and similar operations are necessarily employed to addthrough-thickness fiber orientation and improve interlayer strengthproperties. However, a preform with fully isotropic character in thefiber reinforcement continues to be difficult to attain.

Current methods for producing carbon-carbon fiber reinforced compositesexhibit further shortcomings. For most applications, finished carbonparts generally are made to precise dimensions, and their productionrequires conducting extensive shaping and machining operations oncarbonized or fully graphitized carbon-carbon fiber composite blanks.Precision machining operations are expensive to carry out and difficult,and great care is needed with carbon-carbon fiber composites to avoidcracking or other damage. Carbon blanks having substantially thefinished shape and dimensions, termed net shape blanks, would reduce theextent of machining needed and significantly lower costs. However,carbonized preforms are generally friable and cannot be readily formedor shaped. Constructing shaped preforms from layered fabric or fibersheet thus generally requires cutting component parts having the desiredfinal shape from fabric sheet before stacking and needlepunching. Suchcutting operations are wasteful and produce considerable quantities ofscrap fabric. Even when suitable methods for recycling of the scrap arefound, the production and re-processing of scrap further increases theenergy and waste disposal burdens already imposed on the manufacturingprocess, significantly raising the overall cost of producing the carbonarticle.

Methods for producing carbon-carbon fiber reinforced preform structuresfrom staple or chopped carbon fiber are also disclosed in the art. Forexample, in U.S. Pat. No. 4,297,307 there is described a process forextruding a thickened or gelled slurry or dispersion of cut carbon fiberin liquid medium such as water to form an elongated ribbon. The liquidmedium may include a carbonizable polymeric binder to bind the fibercomponent on drying. The elongated ribbon is then arranged in a circularpattern to form a flat disc, and dried to remove the water. Theorienting effects of the fluid flow in the thickened medium duringextrusion permit aligning or orienting the fiber along the flow line,resulting in very low density, non-woven, discontinuous fiber discs withcircumferential fiber orientation. The dried disc may then be heatedand, if appropriate, carbonized. A plurality of the resulting thin,low-density discs may be stacked to provide the necessary thickness andthen subjected to infiltration or impregnation operations as describedand carbonized or graphitized to produce carbon discs for use in brakesor the like. As with other layered structures lacking interlayer fiberreinforcement, the resulting layered carbon disc will be subject todelamination failure.

Methods for forming non-woven webs of carbon fiber have also beendisclosed in the art, for example in U.S. Pat. No. 4,032,607. Accordingto patentees, particularly attractive webs may be formed from mesophasepitch by melt- or blow-spinning the pitch, air-layering orwater-layering the resulting fiber either as-spun or after beingchopped, and thermosetting or air-oxidizing the non-woven web tostabilize the structure before carbonizing. Generally, the resultingwebs are composed of random filaments rather than filament bundles ortow, and take the form of low density, thin felts and papers with verylow bulk densities, generally well below about 0.3 g/cc. Non-woven websmay be suitable for use in forming layered carbon-carbon fiberstructures in the same manner as continuous fiber tape and fabric byemploying prior art layering and needlepunching operations such as thosedescribed herein above. Even after the needlepunching, structurescomprising such highly randomized filaments generally will have a lowfiber volume and consequently a very low density. Such structures wouldnot provide the strength advantages generally obtained when using dense,high fiber volume structures comprising aligned and oriented continuousfiber, either in woven textile form or as unidirectional fiber tape.

Preform structures fabricated from cut or chopped fiber heretoforeavailable in the art generally are also low in density and lack themechanical strength necessary for use in carbon-carbon fiber composites.Methods for fabricating suitable carbon-carbon fiber composites with ahigh fiber volume from discontinuous fiber are unknown in the art, andthe carbon composite industry is thus forced to rely primarily onpreforms fabricated from oriented and aligned continuous fiber in orderto produce carbon-carbon fiber reinforced composites having the strengthproperties desired for use where high levels of mechanical stress areencountered. Such prior art composites generally also are deficient inheat transfer properties, particularly in the out-of-plane or thicknessdirection, further limiting their utility.

A method for fabricating thick preforms and carbon-carbon fibercomposite blanks having adequate strength properties and good thermalcharacteristics from cut or chopped fiber, preferably in a net shape andavoiding the use of binders and liquid carriers that add further to theenergy and disposal burdens on the manufacturing process, would beparticularly valuable to the carbon composites art.

BRIEF SUMMARY OF THE INVENTION

Dense, porous carbon preforms having a three-dimensional fiber structuresuitable for use as reinforcement in the manufacture of composites andparticularly desirable for use in producing high-strength, high thermalconductivity carbon-carbon fiber reinforced composites may be producedby needlepunching a thick, low-density mat formed of discontinuousthermoset pitch fiber through the thickness, preferably normal to asurface. The needlepunching serves to increase the density of thestructure and to re-orient a portion of the fiber in the thicknessdirection to improve integrity and strength characteristics. The preformmay conveniently be produced directly from fiber as a net-shape preformhaving the general overall shape of the final product, together withdimensions needed to accommodate such shrinkage as may occur duringsubsequent thermal treatment. The net-shape process minimizes the scrapproduction and concommitant waste encountered in prior art processes forfabricating textile fabric, sheet and tape, and reduces the need forextensive machining and forming operations.

The invented preform will generally consist entirely of carbonized fiberand, though made without resort to binders or the like, the mechanicalstrength of the preform will be adequate to withstand subsequent carboncomposite manufacturing operations including infiltration with pyrolyticcarbon or impregnation with a carbonizable filler and subsequentcarbonization. The preform may also have application in the manufactureof carbon fiber-reinforced thermoset resin matrix, metal matrix andceramic matrix composite structures.

A dense carbon-carbon fiber composite may be readily produced bydepositing pyrolytic carbon within the invented preforms usingwell-known chemical vapor deposition processes and infiltrationoperations generally known and widely employed in the composite art.Alternatively, the preform may be impregnated with a carbonizablefiller, cured under pressure and heat, and then further heated tocarbonize the filler together with any pitch fiber component present,thereby providing a dense carbon-carbon fiber composite. Multipleinfiltrating or impregnating operations may be employed if needed toproduce a product having the desired density, and the processes may beused in combination.

DETAILED DESCRIPTION

A porous, thick, three-dimensional carbonized fiber preform suitable foruse in the manufacture of high strength carbon-carbon fiber compositesmay be made from thermoset pitch fiber according to the inventionwithout application of a binder or an impregnant.

Carbon fibers have long been known, and methods for their productionfrom a variety of precursors are well described in the art. Cellulosicprecursors have been used for producing carbon fiber since the early1960's, with rayon being the dominant carbon fiber precursor for nearlytwo decades. More recently, as the an has developed methods forproducing carbon fiber derived from such materials as polyacrylonitrile(PAN) and pitch, the importance of rayon-based carbon fiber hasdeclined. Polyacrylonitrile fiber, when oxidized and carbonized underappropriate conditions, provides tough, high strength, high moduluscarbon fiber, and the overall conversion yield in producing fiber fromPAN is good. Consequently, PAN fiber has been long preferred forfabricating preform structures.

Carbon fiber may also be readily produced from a mesophase pitch byspinning the molten pitch into fiber, oxidizing the pitch fiber byheating in air to form a thermoset fiber, and carbonizing by furtherthermal treatment in the absence of air. As is well known and understoodin the art, the melt-spun pitch filaments are highly ordered structurescomprising elongated, liquid crystal mesophase domains aligned with thefilament axis. On carbonizing, these domains provide carbon or graphiticfiber with a high degree of crystalline order. Such highly orderedpitch-based fiber has generally been recognized as capable of providingcarbon fiber having greater stiffness and higher thermal conductivitythan carbon fiber from other sources, and carbon composites with asimilar combination of properties and low or even negative coefficientof thermal expansion would find wide application. Moreover, thermosetmesophase pitch fiber is carbonized and graphitized in higher yield thanother carbonizable precursor fibers such as rayon fibers, PAN fibers andoxidized PAN fibers, i.e., thermoset pitch fiber undergoes lessreduction in weight when thermally processed. This in turn may lead toreduced shrinkage during carbonizing and graphitizing operations andminimize the concomitant creation of voids and internal stressesnormally encountered with other fiber precursors. For these reasons,thermoset pitch fiber will be found particularly useful and desirablefor use in the practice of this invention.

Preferably the thermoset pitch fiber will be employed in the form of achopped tow or yarn. The fiber length of the chopped tow will be greaterthan 0.5 inch, preferably greater than 1 inch in length, depending uponthe intended thickness of the finished part. Desirably, the fiber lengthwill lie in the range of from about 1 inch to as great as about 6inches, preferably comprising a mixture of varying lengths within thespecified range, with a nominal or average length of about 2 inches.Continuous fiber tow ordinarily comprises a plurality of filaments,usually from 1000 to 20,000 or more and may even exceed 300,000, withthe axially-aligned filaments providing strength in the fiber directionof the tow. The entanglement of the individual filaments making up thetow desirably maintains filament alignment, even when the tow ischopped. Filaments that are very short, particularly less than about 0.5inch to 1 inch in length, tend to become disentangled and cause the towto open up and separate readily into individual filaments, particularlyduring subsequent handling such as in fabrication operations,needlepunching and the like. Conversely, chopped tow having long fiberlengths, particularly lengths considerably greater than the intendedthickness of the preform, are difficult to reorient using needlefeltingtechniques without causing fiber damage.

As will be subsequently described in greater detail, shaped matscomprising loose, discontinous thermoset pitch fiber will be formedusing molds or the like. The filling operation employed will be moreeffective to impart some degree of flow orientation to the chopped towwhen longer tow lengths are used. For optimum benefit, the tow lengthwill preferably be selected to be greater than the intended thickness ofthe preform, and preferably about twice the preform thickness. Whenre-oriented by the needlepunching operation, the longer filaments mayserve to provide greater reinforcement by extending through the mat inthe thickness direction with a portion of the tow length maintaininguseful entanglement with filaments lying in the plane of the mat.

While the fiber may be substantially dry, preferably the thermoset pitchtow will be sized before being chopped using a conventional aqueoussizing formulation, and more preferably the sized tow will be useddirectly from the sizing bath without being dried, thus retaining from20 to 35 wt % of the sizing carrier liquid as moisture. The use of wettow will provide a mat for needlepunching having a similar moisturecontent. Mats with lower moisture contents are bulky and more difficultto densify in the needlepunching operation, while high moisture contentcauses the fiber to be difficult to chop and may cause the fiber tostick to the needle shafts during the needlepunching operations, therebyremoving fiber from the mat and clogging the needle barbs and apparatus.

The thermoset pitch fiber will be fabricated into a porous,three-dimensionally reinforced preform. Preferably, the fiber will beformed into a thick, low-density mat having the general shape of thepart, with the discontinuous fiber oriented within the plane of the mat.Through-thickness reinforcement will then be added in a subsequentneedlepunching operation. The preform will thus be produced in thegeneral shape of the final product, avoiding the need for cutting andshaping operations and thereby minimizing the substantial waste normallyassociated with such steps.

In forming the low-density mat structure for use in the needlingoperation, means such as a cavity mold or similar assembly are suppliedto form the discontinuous fiber into a mat having the desired shape andto constrain the mat during the needlepunching. For example, where theintended use for the carbon article is as a brake disc or similarobject, the means for holding the fiber may take the form of a cavitymold having an annular shape including substantially flat or planarfaces. The mold, which may be formed of any convenient material havingsufficient mechanical strength to support and constrain the fiber mat,will have at least one wall or face open to admit fiber, as well as topermit carrying out the needlepunching operation. Alternatively, themold may be fitted with a removable closure, and the closure, as well asother walls, may be formed of a material that will permit piercing inthe needlepunching operation without loss of mechanical integrity, forexample, a fugitive material such as scrim, perforated or foam sheet orscreen. Cavity molds of appropriate construction described in the artand known for use in holding or fixturing layered structures and stacksformed of segments cut from fabric sheet or the like for needlepunchingoperations may be suitably adapted for use in fabricating preformsaccording to the instant invention.

The wet thermoset pitch fiber tow will be chopped, fed to the moldcavity and distributed uniformly therein at the desired depth to takethe form of the mold cavity, thereby forming a mat of loose,discontinuous fiber. The tow may conveniently be chopped and feddirectly to the mold in a continuous stream while advancing the mold ata speed that will deposit and distribute the fiber without causingunstable stacking. Means such as a chute or the like, as well as guidevanes, may be employed to direct the fiber stream into the mold, andthese may be also be effective to selectively orient the fiber inparticular areas such as nearest a mold wall.

Because of the shape and substantial length of the fiber, the mode ofadding and distributing the chopped fiber tow will serve to orient thefiber to lie generally in the horizontal plane of the mold, much as isfound for the fiber component of non-woven webs obtained in paper-makingprocesses, and little if any fiber will be found oriented in anout-of-plane direction. For the purposes of further discussion, indescribing the resulting structure, the mold plane will be designated asthe x-y or in-plane orientation or direction, and the z orthogonaldirection will be understood to be the direction normal thereto, alsotermed the through-thickness direction.

It is preferred that the chopped tow be deposited in a manner that willprovide uniform areal weight, giving a fiber mat having a density in therange of 0.3 to about 0.6 g/cc, preferably from about 0.4 to about 0.5g/cc. While mats with densities outside these ranges may be useful insome applications, generally low-density mats having densities belowabout 0.3 g/cc are very light, almost fluffy structures. Such matsgenerally require considerable compaction in order to attain the bulkdensities needed for most preform applications, which usually causessevere fiber damage. Mats with densities greater than about 0.6 g/cc aredifficult to achieve without including continuous fiber in the form ofbraided fiber, tape, fabric or the like, thus increasing fabricationexpense. As will be further discussed below, uniformly needlepunchingdense mats and structures comprising continuous fiber is difficult andresults in considerable breakage of fiber and needles, and hence willdesirably be avoided.

Some density variation in the structure may be desirable. Higherdensities at the intended wear surfaces are preferable in order toprovide improved structural strength and integrity, and may also beeffective in helping to reduce friability in the intermediate preformstructure. Inasmuch as oriented fiber permits achieving greaterdensities through improved packing, it may be desirable that asubstantial portion of the fiber nearest the mold wall be given anorientation of approximately 0° with respect to the mold wall.Completely random fiber orientation, particularly close to mold walls,reduces the ability of the fiber to pack well and necessarily results inthe lowest density. Such preforms and the resulting carbon-carbon fibercomposite structures lack adequate strength for many applications,particularly where lugs or other load-beating features are to beprovided at the part edges as an integral part of the composite.

Close fiber packing increases the mat density, effectively reducingporosity and making infiltration more difficult; hence it is preferredthat fiber in the bulk of the structure and away from mold walls lie inthe x-y plane in a more randomized, less oriented configuration and givethe mat more in-plane isotropic characteristics. Preferably, the fiberwill lie in the mold plane and have an average orientation desirably onthe order of ±45°, although orientations as great as ±75° in the bulk ofthe mat may also be found acceptable for use in some less demandingapplications. Fiber orientations greater than about 45° tend to reducecircumferential strength while orientations of less than about 15° tendto lower the radial shear strength of the resulting composite, hencesuch extreme orientations will not be preferred for most applications.Again, the fiber will preferably have a substantially planar orientationwith respect to the x-y plane, the orientation descriptions being setforth and described with respect to the nearest mold wall or walls thatintersect the x-y plane such as, for example, a wall defining theperiphery of an annular or disc preform.

It is important that any variation in the orientation of the chopped townot occur abruptly or discontinuously, thereby creating bands containingfiber with a single orientation, but rather that the transition becontinuous and gradual. Bands comprising fiber having a very narrow orsingle orientation and with few bridging fibers internally or extendinginto adjoining areas may cause severe delamination and other structuralfailures by concentrating stress within a narrow region of the structureand preventing the re-distribution of the imposed stresses throughoutthe structure.

In an alternative embodiment, fiber may be supplied to the mold cavityin continuous form, in a manner that will distribute the continuousfiber filaments uniformly within the cavity and provide a mat having adensity in the desired range. For example, it is known that fiber may bedistributed in layered fashion to form mats by employing a sort ofpiddling motion about the mold central axis when feeding the continuousfiber strand. As noted, the needlepunching operation will tend to breakthe continuous fiber to provide random lengths of tow for reinforcementin the through-thickness direction. Hence the use of continuous fiber insuch constructions will not be preferred.

The low-density mat produced in the initial operation will generallyhave a thickness in the range of from about 1/2 inch to about 4 inches,preferably from about 3/4 inch to about 3 inches for most applications.Mat thickness will be limited in part by practical considerations. Inparticular, the mat is intended to be further modified to providethree-dimensional reinforcement in a needlepunching operation. Suchneedling operations generally require the use of needles of a lengthsufficient to penetrate from 90 to 100% through the thickness of themat. For mats with excessive thickness, the force needed to penetrate tothe desired depth may result in frequent needle breakage. In addition,needles able to penetrate to depths of 4 inches and more are generallynot available from commercial sources and must therefore be custom made,adding to the cost of preform fabrication.

Needling operations are conventional in the non-woven textiles art, andgenerally are practiced using a plurality of needles comprised of ashank having outward-projecting barbs. The needles are mounted to permittheir use in tandem in the needling operation, the needlepunching beingaccomplished by moving the needles normally with respect to the matsurface, and in a reciprocating manner, thereby forcing the needlesrepeatedly into the mat. The barbs catch fiber in passing through themat, causing a portion of the fibers in the mat to become alignedvertically. In practice, the presence of fiber size together with a highlevel of moisture benefits the needlepunching operation by aiding needlepenetration and, perhaps by acting as a lubricant for the fibersubstrate, assisting the reorienting of fiber tow in the needledirection.

For the purposes of this invention, the needles will preferably beselected to be of sufficient length to pierce the low density matsubstantially through, preferably from 80 to 95% through, morepreferably about 90% through, the mat in the needled direction. Theneedle density will be selected to provide vertical fiber orientation atdensities adequate for reinforcement of the preform in the thicknessdirection. In the practice, the needles will be set at spacings of from0.9 to 1.2 cm on center.

Typically, in operation a portion of the filaments making up the choppedtow will be caught by the barbs and reoriented as filament bundles inthe z or thickness direction. The portion of the filaments not caught bythe barbs will be displaced laterally within the x-y plane by theneedles, creating openings through the structure to the depth of theneedling. The needlepunching thus serves to redistribute the fiber byseparating the fiber tow into smaller filament bundles and byrandomizing the fiber orientation within the x-y plane through lateraldisplacement of the filament bundles. The filament bundles created bythe needlepunching will vary widely in number of filaments, dependingupon the initial makeup of the fiber tow and on the level ofneedlepunching employed. Structures comprising filament bundles withfrom 25 to 1000 filaments will be readily produced, while structureswith bundles having as few as 10 to 75 filaments or bundles comprisingfrom 250 to as many as 20,000 may also be observed.

For the purposes of this invention, the needles will be selected to beof a size that will afford vertical openings of significant size,generally of sufficient diameter to accommodate the filament bundlesdisplaced into the opening by being reoriented in the needlepunchingoperation and provide substantial free space surrounding the displacedtow for subsequent infiltration of matrix material. When carbonized, thestructure will then have openings extending substantially through thestructure and will exhibit high porosity. When infiltrated orimpregnated with matrix material, columns of matrix material withembedded filaments will be formed extending substantially through thethickness of the composite. When viewed from the z direction, thecross-sectional area within the columns occupied by matrix material willbe seen to be preferably at least twice, more preferably greater thanfour times, that occupied by the fibers.

By contrast, prior art processes produce needled carbon fiber preformsfrom layered PAN fiber fabric or unidirectional tape by needlepunchingeach layer to the underlying layer. The needles are selected to be thinand are closely-set, often at densities of 30 per square inch andgreater, and are intended to give the least disturbance to the fiberalignment and avoid displacement of the fiber tow, other than thefraction of the fiber that becomes reoriented in the needling. Eachlayer is needled as it becomes superposed on the layer below, and fiberis displaced in the vertical direction, reinforcing only adjoininglayers. Ordinarily in these prior art processes the needlepunching isapplied only to the two adjacent layers, or at the most only a fewlayers are included, and fiber alignment in the thickness or z directionis thus discontinuous. Moreover, the openings in the needled layers thatresult from lateral fiber displacement are minimal and nearly completelyfilled with displaced fiber. When carbonized, the resulting preformswill thus lack passages through the structure and the passages will havelittle free space, and will thereby exhibit a lower porosity. In orderto provide openings in such prior art structures that are continuous andextend through more than the two or three needled layers, it would benecessary to precisely align the needlepunching for each of thesuperposed layers, a very difficult process to carry out successfully.

The needlepunching operation is preferably accomplished by piercing in adirection normal to the surface of the mat structure. However, where thelow density mat structure has a curvilinear outer surface or whereparticular fiber orientations are desired, piercing may be accomplishedat angles other than normal to the surface, and piercing angles as greatas about 45° to the surface may be usefully employed.

To attain greater uniformity in the needled structure, particularly formats and similar substantially planar structures, it may be desirable toneedle punch the structure from both faces by inverting the structureand presenting the opposing face for additional needlepunchingoperations. As will be better understood by way of being exemplifiedherein below, when combined with controlled variation in the depth ofthe needlepunching, the technique of needlepunching from both faces alsoaffords means for controlling the degree of reinforcement within thestructure, thus providing structures having different levels ofreinforcement through the thickness.

The repeated piercing in the needlepunching operation serves to compactand thereby densify the structure to some degree while at the same timespreading, randomizing, and reorienting fiber within the plane of thedisc. This working of the fiber tow further randomizes the fiber,smoothing the progression in fiber orientation between adjacent areasand minimizing discontinuities. As noted, the needles employed areselected to provide openings or channels of significant size through themat, with reoriented tow filaments extending through the channels. Thelarge channels increase the effective porosity of the structure andprovide improved access to the interior of the mat, increasing the levelof carbon deposition that can be accomplished in subsequent carboninfiltration operations or for impregnation with suitable carbonprecursor materials. The improved porosity is particularly beneficial inthe production of thick parts, generally greater than 1 inch, becauseduring the infiltration of low porosity preform structures with thematrix component, such as, for example, CVD carbon, deposition tends tooccur at the surface layers, blocking further densification within theinterior of the structure.

The needlepunching operation will generally reduce the initial thicknessof the fiber mat by from 10 to 40%, depending upon the degree ofneedlepunching applied. For most applications the level of needlefeltingemployed will be sufficient to reorient substantial quantifies of fiber,thereby giving the structure three-dimensional reinforcement and mayresult in isotropic property characteristics. The strength propertiesand preform integrity will be significantly improved throughneedlefelting, permitting subsequent handling including storage,packaging and shipping, as well as use in impregnating and infiltrationoperations without requiring further fixturing.

The needle-punched thermoset pitch fiber preform will generally becarbonized to provide a porous carbon structure or preform for use inthe infiltration or impregnating and carbonizing operations used tocomplete the production of a high-strength, fiber-reinforced carbon orgraphite composite article. Carbonizing processes such as are commonlyemployed in the art for such structures may be used for these purposes.Generally, the preform may be carbonized without the need for fixturingby heating in an inert, non-oxidizing atmosphere at a heating rateselected on the basis of the size of the preform and the materials ofconstruction. Heating rates in the range of from about 25° to 50° C. perhour up to the final temperature are commonly employed in the art, andthe preform may be held at a selected final temperature for varyingperiods of from several minutes to several hours to complete thecarbonization, the time depending upon the degree of carbonizationdesired. Such processes will be familiar to those in the carbon-carbonfiber art. For most preforms from thermoset pitch fiber precursors, thecarbonizing operation will result in a shrinkage in the range of 3-8%.The carbonized preforms generally will have a nominal bulk density inthe range 0.4 to 0.7 g/cc.

In an alternative embodiment, the preform structure may be altered tomeet particular needs by including fiber that will be consumed in thecarbonizing operation, termed fugitive fiber, or by providingnon-fusible fiber or particulates within the intersticies. One methodfor accomplishing such variant structures will be to furnish the matwith a top layer of discontinuous, possibly highly-crimped, fugitivefiber prior to conducting the needlepunching, whereby a substantialportion of the fiber that does become reoriented in thethrough-thickness direction by the needlefelting operation will consistof such fiber. Upon subsequent carbonizing, the fugitive fiber will belost and the porosity thereby increased.

The porous carbon structures of this invention, when embedded in any ofa variety of matrix materials, including thermoset resins, metals,carbon and ceramics, provide particularly attractive composites. A greatvariety of thermoset resin systems and formulations suitable for use asmatrix resins in the manufacture of carbon fiber reinforced compositesare known and readily available from commercial sources, including epoxyresins, cyanate resin, phenolic resins, bismaleimide resins and thelike, as well as mixtures and reactive intermediates based thereon. Mostsuch thermoset resins, when formulated to be liquid at the applicationtemperature and with a viscosity sufficiently low for impregation ofporous matricies, will be found to be useful with the porous carbonstructures of this invention in producing composites. Similarly, methodsand processes for infiltrating porous carbon bodies with molten metals,including copper, aluminum, tin, silver, nickel and the like, as well asalloys such as brass, have been developed and are widely known in thecomposite arts and these methods may be employed with the porous carbonpreforms of this invention for producing metal matrix composites.Methods for accomplishing the infiltration of a variety of porousstructures with ceramic materials and precursors, including silica,silicon carbide and silicon nitride as well as with a variety of othernitrides, oxides and the like are well described in the art, and thesealso may be suitably adapted for use with the carbon preforms of thisinvention to provide ceramic-carbon fiber composites.

When used for producing carbon-carbon fiber composite, the porous carbonstructure will be subjected to infiltration operations, for example, thepyrolytic deposition and infiltration processes commonly employed in thecarbon composite art. Generally, the operations are conventional, andmay be accomplished in any suitable vapor deposition furnace having atemperature range of between about 700° C. to about 1900° C. Forexample, pyrolytic carbon may be deposited from a carbonaceous gas suchas methane, ethane, butane, or propane which disassociates under theinfluence of heat. The carbonaceous gas is preferably diluted with aninert gas, for example nitrogen or argon, to facilitate penetration ofthe article. Generally a ratio of from about 1 part by volume ofcarbonaceous gas to about 10 parts by volume of inert gas is suitable touse. A ratio of from about 1:1 to about 1:6 has been found eminentlyuseful. The carbonaceous gas may be fed into an evacuated furnace and insuch case the diluent gas may be eliminated or the amount of inert gasused can be considerably reduced.

The period of time needed to effectively infiltrate the shaped porouscarbon structure depends upon various factors such as the structure'svolume, density, structural shape, fiber size and fiber orientation aswell as on the flow rate of the gas, the deposition temperature and thefurnace pressure. These variables may be empirically determinedaccording to the common practice in the art for the manufacture ofcarbon composites. After vapor infiltration, the assembly is allowed tocool and, if desired, the process will be repeated to further increasethe carbon content and the density of the carbon composite article.

Alternatively, the porous carbon preform may be pressure impregnatedwith a suitable carbonizable filler material, such as pitch or acarbonaceous resin. The article may then be pressure cured, and, aftercuring, baked using a protective atmosphere of nitrogen at atmosphericpressure. During the baking operation, the temperature of the body isgradually raised from the curing temperature to about 800° C. The rateof temperature increase is largely a function of the size of the articleto be baked. Large articles may require a slower rate of temperatureincrease than smaller articles in order that the temperature be uniformthroughout the article, thus avoiding harmful internal stresses that arecaused by uneven heating of the article. After completion of theimpregnating, curing and baking steps, the shape may again be placedunder vacuum and reimpregnated, cured and baked. The number ofimpregnation, curing and baking cycles is determined by the density thatis desired in the finished article.

After completion of the desired number of impregnation, curing andbaking steps, the article may be carbonized or graphitized. Thermaltreatment may be conducted in a single heating step or in stages to atemperature in the range of 1200°-3500° C. to produce carbonized andgraphitized carbon articles of this invention. The heat treatment willbe conducted in a substantially non-reactive atmosphere to ensure thatthe article is not consumed. The non-reactive atmosphere may benitrogen, argon or helium; however, for temperatures above about 2000°C., argon and helium are preferred. Although the non-reactive atmospheremay include a small amount of oxygen without causing serious harm,particularly if the temperature is not raised too rapidly, the presenceof oxygen should be avoided. In addition, wet yarn structures willproduce an atmosphere of steam when heated, which should be purged fromthe furnace before carbonizing temperatures are reached inasmuch assteam is highly reactive at such temperatures. It may be desirable toinclude boron or similar graphitizing components in the furnaceatmosphere and these will be regarded as non-reactive as the term isused herein.

The heating of the preform may be carried out as a single step processor, alternatively, conducted in a series of steps or stages, withcooling and storage of intermediate materials such as filled preformsand carbonized structures for further processing at a later time. Thefinal temperature of the heat treatment will be determined primarily bythe end use application. For example, where it is envisioned the articlewill encounter extreme temperatures, the heat treatment may be conductedto very high temperatures, 2600° C. and greater, and even totemperatures approaching 3500° C. for applications where a high degreeof graphitization is desired. The heat treatment may be carried out withor without applying external pressure to assist the compaction andafford high density composites.

It will be readily understood by those skilled in the art that theparticular thermal processing to be employed will be determined withrespect to the size and geometry of the part that is being produced. Forlarge parts, heat conduction into the center of the part willnecessarily be slow, and long heating cycles and slow increases intemperature may be desirable.

Although it is within the scope of this invention to produce reinforcedcarbon-carbon fiber or graphitized articles with lower density, forexample under 1.4 g/cc, the preferred density range will lie in therange of from about 1.6 to about 2.1 g/cc. Carbon-carbon fibercomposites according to the invention will have excellent thermalconductivity, due in substantial measure to the use of pitch-based fiberin fabricating the fiber preforms. The particular thermal conductivityobserved will depend in part on the final carbonizing temperature, whichin turn determines the degree of graphitization. When carbonized at atemperature of greater than 2000° C., composites having a densitygreater than 1.6 g/cc and comprising the carbonized preforms embedded ina carbon matrix according to this invention may have a thermalconductivity greater than 80 watts/m °K in the through-thicknessdirection.

The invention will be better understood by consideration of thefollowing specific examples illustrating more clearly the exact mannerin which the processes of the present invention may be carried out. Theexamples are provided by way of illustration only, and are not to beconstrued as limiting the scope of the invention to the particularprocess details or articles illustrated.

EXAMPLE 1

A mold cavity was constructed by forming a 6" diameter circular openingthrough a 2" thick polyethylene foam sheet, closing one end of theopening by affixing a sheet of Graphfoil at one face of the foam sheet,and affixing a 3.5" diameter cylinder of the foam to the Graphfoilwithin the cavity and axially aligning it at the center. The cavity wasfirst lined with polypropylene scrim, then filled uniformly with 282 gof chopped, 4000 filament, thermoset pitch fiber tow having a nominal 2"length to provide a loose fiber mat consisting of chopped fiber towrandomly oriented substantially along the plane of the mold at a nominaldepth of 2". The mat was then needled normally to the surface with 100reciprocating strokes in two passes over the surface, using a needleboard having a randomized pattern of 216 Foster F20 8-32-5B 2B/E 15 1825 3.5 S BA needles affixed at 1 needle per square centimeter. Thethickness of the mat was reduced to 1.75" in the first pass, and to1.56" in the second pass. Additional needling passes, to a total of sixpasses, were carried out, giving a final thickness of 1.375". After thethird needling pass, the preform was inverted to permit needling fromthe reverse side. The porous preform was readily removed from the moldcavity, and had good strength and integrity. The bulk density of thepreform was 0.628 g/cc, corresponding to a thermoset pitch fiber volumefraction of 0.458.

EXAMPLE 2

A disc-shaped mat, 18.5 cm in diameter and 4.5 cm in thickness, wasformed by hand from 700 g of chopped, 4000 filament, thermoset pitchfiber tow having a nominal length of 1.5". The mat surfaces were coveredwith polypropylene scrim and secured about the circumference withpolypropylene scrim held with strapping tape. A needle board having arandomized pattern of 216 Foster F20 8-32-5B 2B/E 15 18 25 3.5 SBAneedles affixed at 1 needle per square centimeter was used to make 1300penetrations per square inch (NPSI) at a depth of 90% of the matthickness. The needle felting operation was applied from both sides,thereby applying a total of 2600 NPSI to the center 80% of thethickness. The disc material spread slightly, making the disc 19.75 cmin diameter and 3.85 cm thick. The disc weighed 701 g and had excellentintegrity and handling.

The disc was carbonized by heating in a nitrogen atmosphere, increasingthe temperature at the rate of 50° C./hr to a final temperature of 1300°C. with a hold time of one hour at that temperature. After trimming to16.5 cm diameter and 3.3 cm thickness, the disc weighed 408 g,corresponding to a bulk density of 0.58 g/cc. The disc was infiltratedby CVD processing to vapor deposit carbon and provide a carbon-carbonfiber reinforced blank. The blank had a density of 1.65 g/cc in threeCVD cycles. After a total of six such CVD cycles, the density was 1.85g/cc.

EXAMPLE 3

A mold was constructed of aluminum to have a ring-shaped mold cavity of13" outer diameter, 4" inner diameter and 2" in thickness, with a flator planar bottom. The mold cavity was filled uniformly with 1440 g of4000 filament, thermoset pitch fiber tow having a nominal 2" length bychopping the tow into the mold cavity, using a spiral feed motion toevenly distribute the fiber and provide a loose fiber mat withdiscontinuous thermoset fiber randomly oriented substantially along theplane of the mold at a nominal depth of 2". The face of the mat wascovered with polypropylene non-woven scrim. A needle board having arandomized pattern of 49 Foster F20 8-32-5B 2B/E 15 18 25 3.5 SBAneedles affixed thereto, the needles being arranged in an annular arcsegment at 1 needle per square centimeter, was used to make 1300penetrations per square inch (NPSI) at a depth of 90% of the matthickness, followed by 1300 penetrations per square inch (NPSI) at adepth of 60% of the mat thickness. The needle felting operation wasrepeated after reversing the mat to expose the opposing face, therebyapplying the needle felting equally from both sides. In each of theneedling operations, the mold was radially-incremented up to 0.1" torandomize the needle penetrations. Registration per stroke was a radialmovement of approximately 1°. A stripper plate fitted with an annularshoe contacting the face of the mat and sized to fit into the moldcavity was also employed. The stripper shoe was lowered into the mold asthe mat became compacted during the needle felting operations.

The differential needling provided a disc wherein the center 20% of thethickness was exposed to a total of 5200 NPSI and the next 30% in eachdirection from the center to a total of 3900 NPSI, while the outer 10%nearest each of the faces received a total of 2600 NPSI. The resultingneedled mat or annular preform had a thickness of 1.25" and a bulkdensity of 0.52 g/cc. The pitch fiber preform was carbonized, theninfiltrated by CVD processing to vapor deposit carbon on the fiber andprovide a carbon-carbon fiber reinforced blank. After a total of threeCVD cycles, the bulk density was 1.79 g/cc.

EXAMPLE 4

A cavity mold similar in configuration to that employed in Example 3 butconstructed from aluminum, PVC and fiberboard to provide a ting-shapedcavity 1.75" in depth with an outer diameter of 20.1" and an innerdiameter of 8.25" was filled with 2" nominal length chopped, 4000filament, thermoset pitch tow. The filling operation was accomplishedusing articulating chutes located at the inner and outer diameters ofthe cavity to receive the tow from the chopper and give the tow floworientation by flowing in contact with the surfaces of the chute bottomand walls. The stream was fed to the mold as the mold was rotated,filling the mold to the desired depth in several passes. A plate in theform of an arc section was used to press the fiber into the mold cavityafter each pass. By visual examination, the fiber lay generally in thex-y plane, the fiber nearest the walls of the mold cavity being alignedwith the wall and becoming progressively randomized away from the moldwalls to achieve a substantially random orientation in areas more than1.5" from the walls.

The mat was needle punched substantially as described in Example 3,giving a handleable three-dimensionally reinforced thermoset pitch fiberpreform. Upon carbonizing, the resulting porous carbon fiber preform hada nominal 0.53 g/cc density. On visual inspection, the surfaces at theinner and outer circumferences of the disc had good integrity, with onlythe tows nearest the walls not integrated into the structure by theneedlepunching operation.

EXAMPLE 5

The process of Example 2 was employed in forming five disc-shaped mats2" in diameter and 1.1" in thickness. The mats were needled to 90% ofthe mat thickness using the needle configurations and procedures as inExample 2, but with varying levels of needlepunching, providing sixthermoset pitch fiber preforms or test discs having needle penetrationlevels of 650, 1300, 2600, 3900, 5200 and 6500 NPSI from each face. Thespecimens were graphitized to a final temperature of 2600° C. to provideporous carbon fiber preforms. The preforms were impregnated with anepoxy resin by resin transfer molding and cured to provide carbonfiber-reinforced epoxy matrix composite blanks for test purposes. As iswell understood in the art, the function of the epoxy matrix componentof resin matrix composites is to serve as a binder for the fibercomponent. The mechanical properties of such composites, andparticularly the thermal and compressive properties, are primarily afunction of the properties of the fiber reinforcement.

Mechanical test specimens were machined from the discs for determinationof compressive modulus in the in-plane (x/y axes) and in the thickness(z axis) directions using strain gauges. The modulus data are summarizedin the following Table I.

                  TABLE I                                                         ______________________________________                                                  x-y Compressive                                                               Modulus    z Compressive Modulus                                    Needlepunch Level                                                                         Ave     Range    Ave                                              (NPSI/side) (Mpsi)  (Mpsi)   (Mpsi)                                           ______________________________________                                         650        2.8     1.6-3.7  0.5                                              1300        1.85    2.7-1.3  0.8                                              2600        1.8     2.0-1.6  1.0                                              3900        1.5     1.6-1.3  1.4                                              5200        0.9     0.8-1.0  2.2                                              6500        0.9     0.6-1.4  2.7                                              ______________________________________                                    

It will be seen that compressive modulus in the z or through-thicknessdirection for these carbon fiber reinforced epoxy composite testspecimens increased linearly with the level of needlepunching,demonstrating the contribution to stiffness provided by the increasedlevel of fiber having an axial or through-thickness orientation. In thein-plane or x-y direction, compressive modulus undergoes a correspondingdecrease as the level of needlepunching increases.

The ability to attain a broad range of fiber orientations ranging fromrandom to substantially isotropic distribution of fiber in a thick fiberreinforced composite structure is unique to the invented process. Theuse of discontinuous fiber having the appropriate lengths, determined asset forth herein, together with needlepunching to reorient andredistribute fiber and thereby provide uniform distribution of theoriented, discontinuous fiber within the structure, provides means forselectively configuring the fiber reinforcement to have any degree oforientation desired. This degree of flexibility will be of particularbenefit for producing carbon fiber-reinforced composites tailored tomeet specific requirements of a particular application. Prior artprocesses in which thick composites are constructed from continuousfiber generally produce quasi-isotropic, layered fiber structures. Evenwhen complex prior art processes and equipment are used forneedlepunching the thick, layered preforms to improve uniformity, theresulting composites lack true isotropic character and are unable toprovide fiber reinforced composites with through-thickness compressiveproperties equal to or substantially greater than compressive propertiesmeasured in the in-plane direction without resort to three-dimensionalweaving or the like.

EXAMPLE 6

A carbon-carbon fiber composite specimen according to the invention wasprepared substantially following the procedures set forth in Example 2.A test specimen cut from the composite was sectioned in the x-y plane toexpose the ends of fiber oriented in the z direction. The specimensurface was polished using a 1.0 micron alumina slurry, then examined byvisual microscopy using reflected light, under crossed nicol prisms at500X magnification. Turning now to consideration of the photomicrographof the surface, FIG. 1, the z direction filaments, the ends appearing assubstantially round dots, will be seen to be embedded in the CVD carbonmatrix material. Further, the filaments will be seen to be distributedin a substantially random fashion, each surrounded by large crystallinecarbon domains. The area ratios for the CVD carbon and for the carbonfilaments will be seen to be very high, generally greater than about 4.

Comparative Example A

A specimen cut from a carbon-carbon fiber composite according to theprior art comprising a needlepunched fiber laminate stack that had beencarbonized and then infiltrated with CVD-deposited carbon was similarlysectioned and polished. Turning to consideration of the photomicrographof the surface of the prior art specimen, FIG. 2, the specimen will beseen to have a very much larger number of z direction filaments, theends appearing as dark dots, distributed almost uniformly throughout thefield, the spaces between the filaments filled with small and irregulardomains of CVD carbon surrounding the filaments. The area ratios for theCVD carbon and for the carbon filaments will be seen to be low,generally less than 2.

Sectioning specimens cut from the invented composites and preforms, suchas those represented by FIG. 1, in the through-thickness or z directionand examining the surfaces microscopically, will show the composites ascomprising large, elongated domains of CVD carbon having embeddedtherein carbonized filaments oriented in the z direction. Further, thecolumns will generally be found to extend substantially through thethickness of the specimen, thereby providing long columns comprisingfiber and carbon in the through-thickness direction of the composite.

EXAMPLE 7

A carbon-carbon fiber reinforced blank, prepared substantially asdescribed in Example 2, was further carbonized by heating in nitrogen toa final temperature of 1800° C. The carbon-carbon fiber composite blankhad a thermal conductivity of 109 watts/m °K, measured at 75° F. in thein-plane (x-y) direction, and a thermal conductivity of 100 watts/m °Kat 75° F., measured in the through-thickness or z direction. The thermaldiffusivity was independently measured as 1.05 cm² /sec.

Comparative Example B

A test specimen was cut from a commercial brake disc formed from priorart carbon-carbon fiber composite comprising a carbonized fabricinfiltrated with CVD-deposited carbon. The test specimen was found tohave a thermal conductivity of 4.6 watts/m °K, measured at 75° F. in thethrough-thickness direction. The thermal conductivity in the in-plane(x-y) direction was 12.8 watts/m °K, measured at 75° F. Thecarbon-carbon fiber composite of the prior art will thus be seen toexhibit a high degree of anisotropy in thermal properties.

EXAMPLE 8

A disc-shaped mat 50.8 cm in diameter and 3.2 cm thick, needled at 2600NPSI, was prepared substantially by the process of Example 4. A plugcutter was used to cut 5/81th inch diameter cylindrical plugs throughthe thickness for use as test specimens. The level of porosity for thetest plugs was approximated for comparison purposes by measuring the airflow through 6.4 cm of sample using a flow meter. The flow test wasconducted by placing two such plugs, linearly disposed end-to-end, in aflow tube. Air was supplied to the flowmeter at an applied pressureregulated at 5 psig. The volumetric air flow through the apparatus wasmeasured as 0.442 liters/min, corresponding to a flow at the 1.979 cm²face of the sample of 0.3619 liters/min. The unit resistance to flow wascalculated to be 0.496 psi/liters/min-cm.

Test specimens with a thickness of 3.9 cm, cut from a disc carbonized toa final temperature of 1300° C. as described in Example 2, had a flow of0.475 liters per minute at 5 psig, corresponding to a flow at the sampleof 0.4286 liters/min and a flow resistance calculated of 0.144psi/liters/min-cm.

Comparative Example C

Plug specimens cut from a 3.2 cm thick carbon-carbon fiber compositeaccording to the prior art comprising a needlepunched thermoset pitchfiber fabric stack needlepunched to a level of 2600 NPSI were tested forporosity level substantially as in Example 8. The flow was measured at0.438 liters/min at 5 psig, corresponding to a flow at the sample of0.3524 liters/rain and a flow resistance calculated as 0.544psi/liters/min-cm. Test specimens with a thickness of 2.6 cm, cut from adisc carbonized to a final temperature of 1300° C. as described inExample 2, had a flow of 0.470 liters per minute at 5 psig,corresponding to a flow at the sample of 0.4286 liters/min and a flowresistance calculated as 0.266 psi/liters/min-cm.

The differences between the prior art fiberous preform structures andthe porous carbon fiber structures of the invention are thus made quiteapparent. The use of large diameter needles and needlepunching throughthe thick structure according to the invented process createssubstantial openings that extend continuously and substantially throughthe thickness of the invented porous carbon preforms, providing a highlevel of porosity and excellent access for infiltration with the matrixcomponent. Thick carbon preforms constructed from layered fabric andneedlepunching through the superposed layers as they are applied astaught by the prior art, when carbonized, will have few continuouspathways through the preform. Such prior art preforms exhibit much lowerporosity and are thus much more difficult to infiltrate.

Composites, and particularly carbon-carbon fiber composites, comprisingthe invented porous carbon structures also will be seen to differsubstantially from prior art carbon-carbon fiber composites. Thecarbon-carbon fiber composite structures according to the invention willcomprise carbon filaments embedded in columns of crystalline carbon thatextend through the structure in the thickness or needlepuncheddirection. As is well-known in the art, the orientation occuring duringcrystallite growth within carbon bodies is greatly affected, if notcompletely determined, by the presence of the substrate on which thecarbon is deposited. The carbon domains shown in the photomicrograph ofFIG. 1 were deposited on carbonized mesophase pitch fiber withcrystallite domains oriented along the fiber axis as noted herein above.The columns of highly-oriented carbon seen in the photomicrographsextend through the mat thickness and similarly will be oriented. Thesecolumns of oriented, crystalline carbon provide the carbon-carbon fibercomposite with substantial graphitic property characteristics and, byextending substantially through the structure, thereby give thecomposite outstanding thermal transfer characteristics in thethrough-thickness or z direction, as is clearly demonstrated by thermalproperties reported for the composites of Example 7.

A remarkable further comparison is afforded by comparing the mechanicalproperties of the epoxy matrix composite of Example 5, constructed fromthe preform needled to a level of 1300 NPSI, and the carbon matrixcomposites of Example 7, also needled to a level of 1300 NPSI. In theepoxy matrix composite, the compressive properties are seen to beanisotropic, the x-y/z compressive moduli having a ratio of 2.31. Thesame carbon preform structure, when infiltrated with CVD carbon to forma carbon-carbon fiber composite as in Example 7, exhibits substantiallyisotropic thermal properties, the x-y/z thermal conductivities having aratio of 1.09.

For comparison, consider the thermal data presented for the prior artcomposite of Comparative Example B, where the thickness or z directionthermal properties are seen to be generally much lower than the in-planeor x-y direction properties. Moreover, these prior art carbon-carbonfiber composites generally exhibit thermal conductivities in thethickness direction considerably less than about 70 watts/m °K, andthermal diffusivities generally below about 0.7 cm² /sec.

Prior art carbon-carbon fiber composites comprising a carbonizedneedlepunched PAN-based fiber fabric stack infiltrated withCVD-deposited carbon are disclosed to have even lower thermaldiffusivities, generally in the range 0.2-0.3 cm² /sec. Also disclosedin the prior art are carbon-carbon matrix composites comprising acarbonized needlepunched pitch-based carbon fiber fabric stackinfiltrated with CVD-deposited carbon, and these also have low thermaldiffusivities, in the range 0.1-0.7 cm² /sec. The thermal history andlevel of graphitizing in these prior art carbon composite parts, and theorthogonal direction of the conductivity measurements, were notdisclosed.

In composites formed from layered continuous fiber carbon fiber fabricor tape according to prior art practices, for example, those shown inFIG. 2, the continuous carbon fiber component of the fabric or tape mayextend substantially through the composite in the x-y plane and thus mayprovide good thermal transfer pathways in the in-plane direction.However, such composites lack continuous crystalline carbon pathwaysextending through the thickness or z direction to provide thermaltransfer pathways, and heat transfer between successive fabric layers ofsuch composites is known to be poor. The thermal characteristics in thez or thickness direction for such composites, as shown by ComparativeExample B, generally will be much lower than for the in-plane direction,giving the composite anisotropic thermal character.

Carbon-carbon fiber composite structures exhibiting substantiallyisotropic thermal properties, particularly at a high level of thermalconductivity, have not been disclosed in the art, and carbon-carbonfiber composites having thermal conductivities in the through-thicknessdirection as great as 70 watts/m °K or greater, or with thermaldiffusivities greater than 0.7 cm² /sec to as great as 1.0 cm² /sec ormore, are unknown in the art. These surprising thermal properties arefound only for carbon-carbon fiber composites produced according to theinvention.

EXAMPLE 9

A carbonized preform was prepared and sectioned to provide testspecimens substantially as described in Example 5. A test specimen,after being further carbonized by heating in an inert atmosphere tofinal temperature of about 1300° C., was infiltrated with molten copperto provide, on cooling, a copper-carbon fiber composite.

Other known processes for infiltration with a variety of matrixmaterials may be conveniently employed with the invented carbon preformsfor providing nickel-carbon fiber composites, silver-carbon fibercomposites or the like. Vapor deposition processes for infiltration withsilicon carbide, metal nitrides and the like are also known in the artand these may also be adapted for use with the invented porous carbonpreforms in providing still further variation in the composite matrixcomponent. Useful composite materials may also be provided by employinga plurality of such techniques sequentially, thereby producingcomposites with matrix components comprising mixtures of metal, carbonand ceramic materials.

The product of the present invention is thus a porous carbon preformhaving three-dimensional reinforcement, the preform being fabricatedfrom discontinuous thermoset pitch fiber by first forming a mat of thecut fiber or tow, then needlepunching to re-orient a portion of thefiber in the through-thickness or z direction. The needled structure isthen carbonized to provide a highly porous carbon preform having a highdegree of porosity; when compared on a flow resistance basis,.carbonized preforms according to the invention having a bulk densitygreater than 0.5 g/cc will have a flow resistance generally below about0.2 psi/liters/min-cm.

The invented porous carbonized preform structures may be employed asreinforcement in the production of composites comprising any of avariety of matrix materials including thermoset resins, metals, carbonor ceramic. Infiltration of the preform with CVD-deposited carbon, orimpregnation with a carbonizable filler and carbonizing, providescarbon-carbon fiber composite articles having excellent thermalconductivity, generally greater than 70 watts/m °K, preferably greaterthan 100 watts/m °K, in the through-thickness direction and a thermaldiffusivity greater than 0.7 cm² /sec, preferably greater than 0.9 cm²/sec. When viewed in the thickness direction, as in a cross-sectionalview taken in the x-y plane, the invented structures generally willcontain carbon filaments embedded in the matrix component, with theratio of matrix to carbon fiber being greater than about 2, preferablygreater than about 4, when determined as a ratio of the cross-sectionalareas for the two components on an in-plane cross-section of thecomposite.

I claim:
 1. In a thick, three-dimensional structure formed of fibrousmaterial in sheet form needlepunched in the thickness direction, theimprovement wherein said fibrous material is in the form of a matconsisting of chopped, multifilament thermoset pitch fiber tow andwherein at least one surface of said structure is needlepunched in thethickness direction to thereby provide structure having a bulk densityof from about 0.4 to about 0.7 g/cc and a plurality of needlepunchopenings extending at least 80% through said structure.
 2. The structureof claim 1 wherein discontinuous thermoset pitch filaments are disposedaxially within said openings.
 3. The structure of claim 1 wherein saidfiber is chopped multifilament tow having a nominal length in the rangeof from about 0.5 to about 4 inches.
 4. The structure of claim 1 whereinsaid preform structure is in annular form, having said needlepunchopenings extending from both faces thereof.
 5. The structure of claim 1wherein said surface has from about 100 to about 10,000 said needlepunchopenings per square inch of said surface.
 6. A three-dimensional porouscarbon structure having a bulk density of from about 0.4 to about 0.7g/cc consisting of carbonized discontinuous thermoset pitch fiber andhaving from about 100 to about 10,000 needlepunch openings per squareinch of surface through at least one surface thereof, said openingspassing at least 80% through said carbon structure and having carbonizeddiscontinuous thermoset pitch filaments disposed axially within.
 7. Athick, three-dimensional structure having a bulk density of from about0.4 to about 0.7 g/cc and consisting essentially of discontinuousthermoset pitch fiber, said structure having a plurality of needlepunchopenings through at least one surface thereof and extending at least 80%through said structure, said openings having discontinuous pitchfilaments disposed axially within.
 8. The structure of claim 7 whereinsaid thermoset pitch fiber is chopped multifilament tow having a nominallength in the range of from about 0.5 to about 4 inches.
 9. Thestructure of claim 7 wherein said structure is in the form of anannulus.
 10. The structure of claim 9 wherein the surfaces having saidneedlepunch openings are the opposing faces of said annulus.
 11. Thestructure of claim 7 wherein said structure has from about 100 to about10,000 needlepunch openings per square inch of said surface.